Thermal‐Optical Performance of the GPS III Laser ......Transient Test ‐sweeping through solar...
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11/7/2018 1 http://space‐geodesy.nasa.gov Thermal‐Optical Performance of the GPS III Laser Retroreflector Array S. M. Merkowitz, J. Crow, J. Esper, E. D. Hoffman, D. Steinfeld, S. Wall NASA Goddard Space Flight Center P. Lyons Sigma Space Corporation J. Griffiths, C. Moore, M. Nurnberger, L. Thomas, L. Willstatter Naval Research Laboratory 21st International Workshop on Laser Ranging November 7, 2018
Transcript of Thermal‐Optical Performance of the GPS III Laser ......Transient Test ‐sweeping through solar...
11/7/2018 1http://space‐geodesy.nasa.gov
Thermal‐Optical Performance of the GPS III Laser Retroreflector Array
S. M. Merkowitz, J. Crow, J. Esper, E. D. Hoffman, D. Steinfeld, S. Wall NASA Goddard Space Flight Center
P. Lyons Sigma Space Corporation
J. Griffiths, C. Moore, M. Nurnberger, L. Thomas, L. Willstatter Naval Research Laboratory
21st International Workshop on Laser Ranging
November 7, 2018
11/7/2018 2http://space‐geodesy.nasa.gov
GPS Laser Retroreflector Array
SLR to GPS will contribute significantly towards improving the accuracy and stability of the International and WGS84 Terrestrial Reference Frames.
GPS will then provide a means to accurately and uniformly distribute this new accuracy to all systems utilizing GPS.
Delivery of at least 22 arrays for GPS IIIF (SV‐11 launch availability in 2026).
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Geodetic Objectives
Achieve a stable geodetic reference frame with accuracy at least ten times better than the anticipated user requirements for positioning, navigation, and timing.
Maintain a close alignment of the WGS‐84 reference frame with the ITRF. Provide a quality assessment capability independent of current radiometric measurements used to determine GPS orbits and clock performance.
Ensure interoperability of GPS with other GNSS’s (GLONASS, Galileo) through a common, independent measurement technique.
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Cross Section Requirement
ILRS Standard for an “Effective Cross‐Section” of 100 million square meters for satellites at GNSS altitudes.https://ilrs.cddis.eosdis.nasa.gov/docs/retroreflector_specification_070416.pdf
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Array Design Overview
Planar Array – 16 inch diameter 48 recessed 1.6 inch diameter Suprasil‐1 cubes Random clocking Slight 0.2 arcsec dihedral angle spoiling Uncoated back sides Front surfaces coated with an antireflection coating optimized for 532nm and 1064nm
Total array weight: 17.5 lbs. Thermally isolated from the spacecraft (standoffs & blanketing)
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Predicted Cross Section/FFDP
Velocity Aberration: 22‐26 μrad
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Incident Angle ‐ Predicted Cross Section
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Engineering Qualification Model (EQM)
EQM Vibration Verification
EQM After Assembly
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Thermal‐Optical Testbed Salient Features 32” ID x 48” L vacuum chamber with roughing, turbo and
cryo pumps 16 inch clear aperture window Controlled thermal environment via blanketed and painted
shroud, actively cooled (LN2) and heated Solar simulator providing 12 inch diameter 0.8 suns through
window Thermal monitoring via thermocouples Simulate orbit orientation (variable AOI) via internal vertical
axis rotation stage driven by a stepper motor 532 nm CW, power stabilized laser source ‐ currently linear
polarized Source plate optics feeding a 16 inch off‐axis parabola
(OAP) providing a collimated 16 inch beam through the chamber window
Return beam focuses onto a 10x objective feeding a 3.69 micron pixel CCD
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Testbed Optical Layout
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Individual Cube Models and Measurements
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Ambient FFDP Measurements
0° Laser AOIMean OCS = 152x106 m2
14° Laser AOIMean OCS = 119x106 m2
16° Laser AOIMean OCS = 109x106 m2
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Mean Cross Section vs Laser Angle of Incidence
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20.0
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0 2 4 6 8 10 12 14 16
Optical Cross Sectio
n (106
m2 )
Angle of Incidence (deg)
OCS vs AOI
Preliminary result ‐ some pixel count to OCS calibration issues remain
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FFDP vs Solar AOI (Steady State)
0° AOIMean OCS = 151x106 m2
19° AOIMean OCS = 187x106 m2
25° AOIMean OCS = 161x106 m2
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Mean Cross Section vs Solar AOI (Steady State)
Preliminary result ‐ some pixel count to OCS calibration issues remain
Solar breakthrough occurs around 19°
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0 15 19 25 45
Optical Cross Sectio
n (106
m2 )
Solar Angle of Incidence (deg)
OCS vs Solar AOI
9‐Aug
30‐Aug
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Conclusions
Demonstrated the GPS‐LRA meets the ILRS cross section requirement over the range of SLR incidence angles.
The cross section remains stable throughout the full range of solar incidence, including past the breakthrough angle.
Full thermal‐optical performance verification will be performed over the coming weeks, including:Cold Test ‐ soak the array to ‐18°CTransient Test ‐ sweeping through solar beam at orbital speedsDeliberate Gradient Test ‐ thermal gradients generated across the LRA with heaters
